1. Field of the Invention
The present invention relates to a receiver that can selectively receive either a digital broadcasting signal or an analog broadcasting signal.
2. Description of Related Art
Systems for delivering both digital broadcast and analog broadcast at the same time have been proposed. For example, in accordance with an IBOC (In Band On Channel) system proposed by iBiquity Digital Corporation in the United States, a digital broadcast signal is transmitted via a channel associated with an analog broadcast. In this case, the digital broadcasting signal is transmitted with lower power than that with which an analog broadcasting signal is transmitted so that no influence is exerted upon the analog broadcasting signal. In other words, both an analog broadcasting signal having a large amplitude and a digital broadcasting signal that consists of a number of carriers each having a small amplitude exist together in a channel (i.e., a frequency range) located in the vicinity of the centre frequency of the analog broadcasting signal with respect to frequency. A prior art receiver that can receive broadcasts provided such an IBOC system receives an RF signal including both a digital broadcasting signal and an analog broadcasting signal, and a common tuner unit of the receiver converts the RF signal into an intermediate frequency signal by using an RF amplifier, a mixer, an intermediate frequency filter, and so on. This intermediate frequency signal is supplied to both a digital demodulating unit and an analog demodulating unit, and also becomes a feedback signal applied to an AGC or gain control circuit included in the RF amplifier of the tuner unit. As previously mentioned, digital broadcasting signals have amplitudes different from those of analog broadcasting signals, and a problem with the prior art receiver is therefore that the AGC circuit cannot carry out proper gain control because an intermediate frequency signal that contains both a digital broadcasting signal and an analog broadcasting signal is fed back to the AGC circuit.
Japanese patent application publication (TOKKAI) No. 2002-26756 (see paragraph number “0111”) discloses a broadcast receiver for properly controlling both a gain for digital broadcasting signals and a gain for analog broadcasting signals. The broadcast receiver is provided with an amplitude measurement unit for measuring a first amplitude value indicating the amplitude of an analog broadcasting signal and a second amplitude value indicating the amplitude of a digital broadcasting signal, and for outputting both the first amplitude value and the second amplitude value, and a broadcasting system determining unit for determining whether a broadcasting signal selected by an RF amplifier is an analog broadcasting signal or a digital broadcasting signal based on an intermediate frequency signal obtained from an RF signal including the analog broadcasting signal and the digital broadcasting signal. The amplitude measurement unit measures the first amplitude value of the intermediate frequency signal in a frequency band including the centre frequency of the intermediate frequency signal, and measures the second amplitude value of the intermediate frequency signal in a frequency band that is placed off center from the centre frequency of the intermediate frequency signal. The broadcast receiver then selects either the first amplitude value or the second amplitude value according to an output of the broadcasting system determining unit, and generates an AGC voltage based on the selected amplitude value.
However, in some cases, the prior art receiver cannot properly determine whether a broadcasting signal selected by the RF amplifier is an analog broadcasting signal or a digital broadcasting signal under the influence of interfering waves because the receiver is so constructed as to carry out the determination processing based on an intermediate frequency signal. In other words, there is a possibility that even if the digital broadcasting signal has a smaller amplitude than a predetermined value, the digital broadcasting signal apparently has a larger amplitude under the influence of interfering waves or the like, and the prior art receiver then determines that it can receive the digital broadcasting signal. Furthermore, the prior art broadcast receiver can accept both a digital broadcasting signal and an analog broadcasting signal associated with the same television program, and select the digital broadcasting signal under good electric wave reception conditions. When the electric wave reception conditions become worse, the prior art broadcast receiver can automatically switch to the reception of the analog broadcasting signal that serves as backup. However, the prior art broadcast receiver cannot carry out an appropriate switching operation under the influence of interfering waves or the like.